Chang Xiong

Improvement of Light Extraction From Patterned Polymer Encapsulated GaN-Based Flip-Chip Light-Emitting Diodes by Imprinting

To improve surface light extraction of GaN-based flip-chip light-emitting diodes (FC-LEDs), we employed an imprint approach of thermosetting polymer for patterning microscale surface grating on the polymer encapsulant. One-dimensional (1-D) and two-dimensional (2-D) taper-like polymer gratings with a period of 6 mum were successfully realized on encapsulant above the sapphire backplane of GaN LED. By adopting the 1-D and 2-D taper-like grating encapsulant, the improvement of light extraction from the 1 mm times 1 mm FC-blue LED with a reflective Ag film on the p-side was about 18.5% and 31.9% compared to the LED encapsulated by flat polymer, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on 1-D rigorous coupled wave analysis with the supporting experiments.

GaN-based light-emitting diodes with photonic crystals structures fabricated by porous anodic alumina template

In this paper, we propose and demonstrate a convenient and flexible approach for preparation large-area of photonic crystals (PhCs) structures on the GaN-based LED chip. The highly-ordered porous anodic alumina (AAO) with pitch of wavelength scale was adopted as a selective dry etching mask for PhCs-pattern transfer. The PhCs with different pore depths were simultaneously formed on the entire surfaces of GaN-based LED chip including ITO, GaN surrounding contacts and the sidewall of the mesa by one-step reactive ion etching (RIE). The light output power improvement of PhCs-based GaN LED was achieved as high as 94% compared to that of the conventional GaN-based LED.

Light transmission from the large-area highly ordered epoxy conical pillar arrays and application to GaN-based light emitting diodes

To improve the light transmission from the surface, we report a facile and cost-effective approach for the formation of wavelength-scale conical pillar arrays on a large surface area of epoxy resin. The highly ordered epoxy conical pillar arrays with a pitch of about 460 nm and height of about 800 nm have been successfully fabricated by the technique of shape-controlled anodization of Al foil followed by hot embossing. By replicating the tapered pore arrays onto a transparent semi-cylindrical epoxy structure, the incident angular resolved light transmission of the epoxy conical pillar arrays has been obtained. The integrated transmission of conical pillar arrays as high as 62.2% has been achieved which is confirmed to be 223% and 11.3% higher than that of planar epoxy and the cylindrical pillar arrays, respectively. It is reasonable to consider the wavelength-scale conical pillar array as a particular multilayer consisting of a series of two-dimensional photonic crystals with gradually increasing filling factor towards the surface. It can therefore be treated as a multilayer with continuously reducing refractive index towards the air. The conical and cylindrical pillar arrays of epoxy have been directly employed as the encapsulant of a GaN based flip-chip LED. Compared to the LED encapsulated by planar epoxy, the enhancement of light extraction from the LED covered with conical and cylindrical pillar arrays have been demonstrated to be 46.8% and 34.9%, respectively.

Diffracted transmission effects of GaN and polymer two-dimensional square-lattice photonic crystals

The effects of the lattice pitch of GaN and polymer square-lattice photonic crystal (2PhC) on the diffracted transmission were studied by using rigorous coupled wave analysis (RCWA). Besides the first-order Bragg diffraction, higher-order diffractions from large pitches of PhC are also significant to the light extraction improvement. Three different diffraction mechanisms are illustrated through wave vector analysis. The enhancement factors of integrated transmission are obtained from a wide range of pitches with micro-scale for both GaN and polymer 2PhC. The experimental angular-resolved transmission on a transparent polymer sample of 2 microm pitch 2PhC is comparable to the simulation.

High-reflectivity GaN/air vertical distributed Bragg reflectors fabricated by wet etching of sacrificial AlInN layers

Microstructures containing GaN/air distributed Bragg reflector (DBR) regions were fabricated by a selective wet etch to remove sacrificial AlInN layers from GaN-AlInN multilayers. The epitaxial multilayers were grown on free-standing GaN substrates, and contained AlInN essentially lattice matched with GaN in order to minimize strain. Two geometries were defined for study by standard lithographic techniques and dry etching: cylindrical pillars and doubly anchored rectangular bridges. Microreflectivity spectra were recorded from the air-gap DBRs, and indicated peak reflectivities exceeding 70% for a typical 3-period microbridge. These values are likely to be limited by the small scale of the features in comparison with the measurement spot. The stopband in this case was centred at 409 nm, and the reflectivity exceeded 90% of the maximum over 73 nm. Simulations of reflectance spectra, including iterations to layer thicknesses, gave insight into the tolerances achievable in processing, in particular indicating bounds on the parasitic removal of GaN layers during wet etching. Air-gap nitride DBRs as described can be further developed in various ways, including adaptation for electrostatic tuning, incorporation into microcavities, and integration with active emitters.

Improvement of light extraction from micro-pattern encapsulated GaN-based LED by imprinting

We have demonstrated an improvement of light extraction from GaN based flip-chip LEDs by patterning encapsulant. Two dimensional (2D) micron-scale patterns of encapsulant were realized by using imprint technique of thermosetting polymer. This approach has several advantages such as technical simplification, low cost and freedom of material choice. In this work, we fabricated 2D micron-scale patterns with the triangular or sinusoidal profiles on the polymer encapsulated GaN-based flip-chip LEDs. The enhancement factors of light extraction of GaN LEDs with the patterned encapsulant comparing to the flat encapsulated LEDs are about 32% and 47% corresponding to the triangular and sinusoidal profiles, respectively. To evaluate the concept of a diffraction grating in enhancement of light extraction, we performed a simulation of diffraction based on simplified one-dimensional (1D) rigorous coupled wave analysis (RCWA). The calculation reveals that the grating of sinusoidal profile has greater transmittance than that of triangular profile which is in the same trend with the experimental results. These results provide a guideline for improvement of the LED light extraction.

Two-dimensional photonic quasicrystal on the surface of GaN-based light emitting diodes

Two-dimensional (2D) photonic crystal (PhC) patterns in regular four-fold symmetry (2PhC) and dodecagonal symmetric photonic quasicrystal (12PQC) with 970 nm lattice pitch have been formed by electron beam lithography on the surface of GaN-based light emitting diodes (LEDs). The symmetry of a 2D quasicrystal in this work was based on a square–triangle tiling system. The enhancement of light extraction was obtained from both the 2PhC and 12PQC introduced GaN-based LEDs, and 12PQC is favourable compared to 2PhC. To explore the physical properties, we proposed a method for inspecting diffraction behaviours of the PhC arrays. The results supplied a clear representation of the diffraction behaviours of 2PhC and 12PQC that correspond to their lattice symmetry.